1. Field of the Invention
The invention relates to a control device and a control method for a vehicular power system including an induction motor as a prime mover for driving a vehicle.
2. Description of Related Art
A vehicular power system including an electric motor as a prime mover for driving a vehicle is known. For example, a power system for a hybrid vehicle, which includes an electric motor and an internal combustion engine as prime movers, and a power system for an electric vehicle, which includes only an electric motor, are known.
If electric power for driving the vehicle is supplied to the electric motor while the vehicle is hardly moving, the motor may be brought into a state in which electric current keeps flowing into one phase of the motor. The state in which the current is concentrated in one phase will be denoted as “locked state”. If the motor is brought into the locked state, heat may be intensively generated in a circuit that supplies current to the phase in which the current is concentrated, and the circuit may be overheated. Technologies for preventing the overheating are described in patent documents as indicated below, for example.
According to a technology as described in Japanese Patent Application Publication No. 2007-329982 (JP 2007-329982 A), when the vehicle is kept stopped on an uphill road for a long time, through control of the accelerator pedal by the driver, the driving force of the motor is reduced, so as to prevent overheating of the motor that is in the locked state (see paragraph [0002]). It is also proposed to change the degree of reduction of the driving force, according to the gradient of the road surface, required driving force, operating state of the motor, and so forth (see paragraph [0005]).
According to a technology as described in Japanese Patent Application Publication No. 2010-115059 (JP 2010-115059 A), the motor is controlled so that torque becomes equal to or smaller than the maximum torque with which heat generated in the circuit when the motor is in the locked state is within a permissible range (see paragraph [0026]).